Austenitic stainless steel

ABSTRACT

AN AUSTENITIC STAINLESS STEEL CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, .20 MAX. CARBON, 8 MAX. MANGANESE, 1 TO 4 SILICON, 20 TO 30 CHROMIUM, 6 TO 18 NICKEL .50 MAX. NITROGEN, UP TO .02 BORON, AT LEAST 0.03% OF A STRENGTHENING ELEMENT SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM, ZIRCONIUM AND TITANIUM EACH IS AN AMOUNT FROM 0 TO .3% AND THE BALANCE IRON.

July 23, 1974 T. M. COSTELLO ETAL 3,825,417

AUSTENITIC STAINLESS STEEL 4 Sheets-Sheet 1 Filed April 21, 1972 FIG.

} Cr -20/v/ w w 2 2 SE E 3% R55:

2/ 22 23 CHROM/UM CONTENT July 23, 1974 T. M. COSTELLO ETAL 3,825,417

AUSTENITIC STAINLESS STEEL 4 Sheets-Shoot 2 Filed April 21, 1972 0 m S\ 5 3% k Imm NICKEL CONTENT (%l July 23, 1974 T. M. COSTELLO E A 3,325,417

AUSTENITIC STAINLESS STEEL 4 Sheets-Shoot 15 Filed April 21. 1972 FIG 3 I E l E s 5 A M w m i N 3 N 0 T 0/ 0 d I w a C 0 o m 1 2 2 F I H H I 7 I m IV H I 0 0 0 0 0 0 .0 0 0 0 0 0 0 0 0 a 7 6 w 4 w m m w m m ALLOY ADDITION (%I 20 Cr BASE COMPOSITION July 23, 1974 "r. M. COSTELLO ETAL 3,825,417

AUSTENITIC STAINLESS STEEL Filed April 21,

4 Sheets-$heet 4 Fla 4 E\ 5 $5 QERSE QE 353 R q mgwmmfi $61 8w mwtw 3g ktm x BASE MANGANESE CONTENT United States Patent 3,825,417 AUSTENITIC STAINLESS STEEL Thomas M. Costello and Jerome P. Bressauelli, Pittsburgh, Pa., assignors to Crucible Inc., Pittsburgh, Pa.

, Filed Apr. 21, 1972, Ser. No. 246,342

Int. Cl. C22c 39/20 US. Cl. 75-128 A 2 Claims ABSTRACT on THE DISCLOSURE An austenitic stainless steel consisting essentially of, in weight percent, .20 max. carbon, 8 max. manganese, 1 to 4 silicon, 20 to 30 chromium, 6 to 18 nic-kel, .50 max. nitrogen, up to .02 boron, at least 0.03% of a strengthening element selected from the group consisting of columbium, zirconium and titanium each in an amount from 0 to .3% and the balance iron.

As part of continuing efforts to improve the environment by minimizing emissions from vehicle exhaust gases automobiles will be constructed with special emission control devices. In these systems it is useful to employ a thermal reactor wherein the exhaust gas is subjected to additional combustion with air prior to discharge to the atmosphere. During this combustion reaction very high temperatures on the order of 1900 F. are produced within the reactor. Consequently, highly oxidation resistant alloys with good elevated temperature strength are required for the hot zone or core of the reactor. Conventionally expensive, high-nickel austenitic stainless steels, such as AISI Types 310 with 20% nickel, or 330 with 35% nickel, have been the only alloys suitable for providing this combination of properties. Lower nickel alloys, such as AISI Type 309, with 14% nickel, in these applications exhibited inadequate oxidation resistance and strength at the high temperatures involved.

It is accordingly the primary object of the present invention to provide an austenitic stainless steel having high oxidation resistance and good elevated temperature strength during operation at high temperatures on the order of 1900 F.

A more specific object of the invention is to provide an austenitic stainless steel having the combination of oxidation resistance and strength at high temperatures with a relatively low nickel content.

These and other objects of the invention, as well as a complete understanding thereof, may be obtained from the following description, specific examples and drawings, in which:

FIG. 1 is a graph showing the effect of chromium on the oxidation or scaling resistance of chromium-nickel austenitic stainless steels at temperatures of 1900 and 2000 F. and exposure times on the order of 200 hours;

FIG. 2 is a graph showing the eifect of nickel on the oxidation or scaling resistance of chromium-nickel austenitic stainless steels at temperatures of 1900 and 2000 F. and exposure times on the order of 200 hours;

FIG. 3 is a graph comparing the effects of silicon and chromium on the oxidation or scaling resistance of chromium-nickel austenitic stainless steels at temperatures of about 2000 F. and exposure times of about 200 hours; and

FIG. 4 is a graph showing the eifect of manganese on the oxidation or scaling resistance of chromium-nickel austenitic stainless steels containing silicon within the range of the present invention.

It has been found that the above objects may be achieved by an austenitic stainless steel within the weight percent limits set forth in Table I.

3,825,417 Patented July 23, 1974 "ice TABLE I Element Broad range Preferred range Carbon 0.20 max .15 max. Manganese- 8.0 max 3,0 max.

Each up to 0.30 with a Each up to 0.30 with a @flfifitotal of at least 0.03. total of at least 0.03. Iron Balance Balance.

With the preferred range nickel may be further restricted to 10 to 18 percent in combination with nitrogen within the range of 0.05 to 0.20%. Also, within the pre ferred range manganese may be present within the range of 3 to 8%, nitrogen may be present within the range of 0.15 to 0.50%.

Preferably a restricted boron content of .001 to .004% may be used in combination with one or more of the strengthening elements columbium, zirconium and titanium. Ideally, as will be discussed and shown hereinafter, the best results are achieved with a boron content within the range of .001 to .004% and columbium within the range of .1 to .3%.

In establishing the superiority of the alloys of the present invention with respect to conventional alloys and specifically AISI Types 309', 310 and 330 with respect to scaling or oxidation resistance at high temperatures samples of the steels listed in Table II were subjected to cyclic oxidation tests at temperatures of 1900 and 2000 F. for times up to 200 hours. The results are presented in Table H.

TABLE II Weight; gain (mg./in. after 200 .Alloy Cr Ni Si Mn N 1900 F. 2000 F.

It may be seen from these test results that the steels in accordance with the present invention exhibited, at the high temperatures involved, better oxidation resistance than conventional Type 309 having similar nickel content and the oxidation resistance of the alloys of the invention was comparable to the conventional higher nickel Types 310 and 330. In Table III Alloys 2374 and 1G8l with the compositions as listed in Table II were compared with the conventional alloys listed in Table II with respect to elevated-temperature strength. Specifically, in this regard elevated-temperature tensile properties were determined at a temperature of 1800 F. and the results are reported in Table III. In addition creep test specimens of the alloys were prepared and subjected to testing at 1700 'F. at a stress of 1500 p.s.i. for 100 hours. The amount of creep elongation resulting from the testing is presented in Table III.

TABLE III Tensile properties at 1,800 F.

.2% Creep resistance ofiset at 1,700 F. Tensile yield Elongapermanent destrength, strength, tion in formation in Steel k-S-l k.s i. 2 inches 2 in. (percent) "As may be seen from Table III'the' elevated temperature, tensile and yield strengths, ductility and creep resistance of the alloys in accordance with the present invention were superior to conventional alloys.

With respect to the criticality of specific elements of the composition of the invention in achieving the improved properties reported above, reference should be made to the FIGURES and also to Table IV which gives the compositions of the experimental alloys on which the'FIGS. are based. Specifically in this regard FIG. 1 shows-,with a relatively low-nickel alloy in accordance with the present invention that a chromium content in excess of 20% and preferably within the range of 21 to 26% provides good oxidation resistance in air at temperatures as high as 2000 F. Although chromium contents above 24% and 26% are beneficial corresponding increased amounts of nickel must in such cases be added to offset the ferrite forming effect of chromium to insure a fully austenitic structure, which adds considerably to the cost of the alloy.

TABLE IV Composition, weight percent Mn Si Cr Ni N Al 13 Cb Zr The effect of silicon, particularly as a substitute for a portion of the chromium in conventional AISI Types 309 and 310 in promoting resistance to scaling and oxidation at high temperatures is shown in FIG. 3. In FIG. 3 the weight gain is shown for conventional Types 309 and 310 and for compositions restricted to the present invention wherein a portion of the chromium present in conventional grades has been replaced by silicon. This result is achieved without increasing the amount of nickel over that needed to provide a fully austenitic steel. For this purpose silicon contents of about 2% are preferred and amounts substantially over 3 or 4% should not be used because such creates the possibility of cracking during welding and hot working. I

Nitrogen is present as an austenite stabilizer and also promotes elevated temperature strength, as may be seen from Table V.

4 "The amount of manganese preferably depends upon the nitrogen content and, as shown in .FIG. 4, to achieve optimum oxidation resistance at high temperature the manganese content should be maintained as low as possible; however, if nitrogen is increased above about .15% or .20%, manganese'should preferablybe' correspondingly increased upto an amount of about 8% to provide for adequate nitrogen solubility during solidification. For the reasons advanced above, although it is desirable to restrict nickel from the cost standpoint a minimum nickel content of about 8% is necessary toprov'ide structural balance and stability required because of the intended high temperature uses of the steel. With applicants critical combination of alloying elements, however, nickel contents in excess of about 18% as shown in FIG. 2 and needed for conventional chromium-nickel austenitic stainless steels are not necessary for the purpose of achieving the combination of elevated temperature strength and oxidation resistance in the steel of the presentinvention. i The strong carbideand/or nitride-forming strengthening elements columbium, zirconium and titanium may be added alone or in combination to stabilize the steel against intergranular corrosion and to control grain size. When combined with boron these additions, result in improved elevated temperature strength. As may be seen from Table VI creep rupture tests of steels containing various of these elements with boron additions were performed atf1700f under a stress of 2000 p.s.i. for 100 hours. The permanent creep deformation results are reported in TableVI, It should be noted that allsteels were annealed at 200010 2150 F. for 10 minutes prior to creep rupture testing to achieve similar grain sizes of ASTM 4 to 5.

1 Nominal base composition:

It may be seen from Table VI that in accordance with the composition limits of the present invention beneficial effects are achieved with boron alone and in combination with the listed strengthening elements. Addition of titanium in amounts up to 0.30% is also beneficial. In this regard a drastic improvement is achieved by the use of boron in combination with columbium. Boron contents should preferably not exceed .005 to assure good weldability and sensitization resistance.

The claims: I

1. An austenitic stainless steel consisting essentially of, in weight percent, 0.15 max. carbon, 3 to 8 manganese, 1 to 3 silicon, 21 to 26 chromium, 8 to 18 nickel, 0.15 to 0.50 nitrogen, 0.0005 to 0.01 boron, 0.1 100.3 columbium and the balance iron. t

2. The steel of claim 1 having boron within the range of 0.001 to 0.004%.

References Cited UNITED STATES. PATENTS 3,607,239 9/1971 Mimino 128 3,306,736 2/1967 Rundell 75128F 2,590,835 4/1952 Kirkby 75128 F 3,284,250 11/1966 Yeo 75128 A 3,107,997 10/1963 KOZlik 75-128 F 3,615,368 6/1968 Baumel 75-128' C HYLAND BIZOT, Primary Examiner 11.5. C1. X.R.

75--128 C, 128 F, 128 N 

